1. Field of the Invention
The present invention relates to a semiconductor device and a method for fabricating the same. More particularly, the present invention relates to a transistor structure of a memory device having advantages of a recessed transistor and Fin transistor, and a method for fabricating the same.
2. Description of the Prior Art
Recently, as semiconductor devices have been highly integrated, channel lengths and channel widths of the semiconductor devices have significantly shortened. For this reason, it is difficult for conventional two-dimensional transistor structures to ensure sufficient data retention times due to the junction leakage current caused by high-density doping. In addition, the conventional two-dimensional transistor structures present limitations in view of current drivability of the highly integrated memory devices.
FIG. 1 shows a conventional recessed transistor structure, which has been suggested to ensure sufficient data retention time for a highly integrated memory device. In FIG. 1, reference numerals 1 to 4 represent a semiconductor substrate, a field oxide layer, a gate insulation layer, and a gate electrode, respectively. In addition, reference characters S and D represent source and drain areas, and reference character C represents a channel area, respectively.
According to the conventional recessed transistor structure shown in FIG. 1, a recess having a predetermined depth is formed in the semiconductor substrate 1 in such a manner that the source/drain areas can be maximally spaced from the channel area.
If the recessed transistor is used as a transistor of a memory device (for example, a DRAM), the junction leakage current of the memory device can be significantly reduced in comparison with that of a memory device employing a two-dimensional transistor as a transistor. Accordingly, the recessed transistor ensures sufficient data retention time, which may be twice as long as the conventional data retention time.
However, the conventional recessed transistor structure presents problems in that it causes a higher back bias dependency of a threshold voltage and inferior current drivability.
FIGS. 2A and 2B are views illustrating a conventional Fin transistor structure configured to improve current drivability of a highly integrated memory device, wherein FIG. 2A is a perspective view of the conventional Fin transistor structure and FIG. 2B is a sectional view taken along lines X-X′ and Y-Y′ shown in FIG. 2A. In FIGS. 2A and 2B, reference numerals 11 to 14 represent a semiconductor substrate, a field oxide layer, a gate insulation layer, and a gate electrode made from polysilicon, respectively. In addition, reference characters S and D represent source and drain areas and reference characters C1 to C3 represent channel areas, respectively.
According to the conventional Fin transistor structure shown in FIGS. 2A and 2B, a channel area (that is, an active area 11a) of the semiconductor substrate 11 is vertically protruded and the gate insulation layer 13 and the gate electrode 14 are sequentially formed on the active area 11a. Thus, three surfaces (C1, C2 and C3 of FIG. 2b) of the substrate 11 surrounded by the gate electrode 14 can be used as channels of a Fin transistor.
If the Fin transistor is used as a transistor of a memory device (for example, a DRAM), the amount of current flowing through the Fin transistor may increase because three surfaces of the Fin transistor can be used as channels so that the current drivability of the memory device can be significantly improved. In particular, since the Fin transistor contains superior ON-OFF characteristics while lowering the back bias dependency of a threshold voltage, the Fin transistor may represent a high speed memory device with superior device characteristics at a low voltage.
However, the conventional Fin transistor structure presents a fatal problem because it cannot ensure sufficient data retention times due to a structural problem thereof. In other words, it is predicted that a source of the junction leakage current may be significantly enlarged due to the triple channels formed in a narrow area.
Moreover, if the gate electrode is employed in a low-resistant gate electrode structure required for a highly integrated memory device, in which the low-resistant gate electrode structure further includes a low-resistant conductive layer made from WSix or W and deposited on a polysilicon conductive layer, not only is it difficult to ensure sufficient data retention times, but also the resistance of the gate electrode is suddenly increased due to defects (for example, voids) created when fabricating the low-resistant gate electrode structure, thereby causing additional problems.
Such defects may occur when depositing the low-resistant conductive layer made from WSix or W onto the polysilicon conductive layer because there is a step difference between the active area 11a vertically protruding from the semiconductor substrate 11 and the field oxide layer 12.